Recommendations from professional societies
The American Diabetes Association recommends blood glucose testing as soon as possible in high-risk women and routinely at 24 to 28 weeks gestation in those at lower risk.8 The American College of Obstetricians and Gynecologists (ACOG) does not address urine testing for glucose.9 The Institute for Clinical Systems Improvement (ICSI) considers urine dipsticks for glycosuria unreliable.10
Abandoning all but the initial urinalysis may miss a few women with true but unrecognized diabetes mellitus. None of the studies presented above address this problem although screening for diabetes mellitus using urine test strips is not an ideal screening test, identifying only between 30% and 59% of a predominately middle-aged nonpregnant group.11
There is no evidence that testing for gestational diabetes before 28 weeks, as might be prompted by urine testing, changes pregnancy outcome. Screening for gestational diabetes by glycosuria is not effective with low sensitivities and low positive predictive values. False-positive tests outnumber true positives 11:1, leading to unnecessary further testing. Based on the information available, it appears safe to abandon routine urine testing for glucose at every prenatal visit. This recommendation stands regardless of the debate over the value of screening for gestational diabetes by 50-g glucose challenge followed by an OGTT if indicated.12
Proteinuria as a predictor for preeclampsia
Proteinuria in pregnancy is common. One study of 913 women reported that 3.8% of them had proteinuria by automated dipstick testing on their first antenatal visit and 40.8% had dipstick-positive (≥1+) proteinuria at least once during the course of their pregnancy.13 In another study of 3122 otherwise healthy women with a single gestation, 9.8% of the women had at least 1 episode of dipstick proteinuria ≥30 mg/dL (≥1+).5
Detection of proteinuria in hospitalized hypertensive pregnant women by visual reading of dipsticks, as is the usual office practice, has a high false-positive rate for true proteinuria (≥300 mg/L) with a PV+ (true positives/true plus false positives) of 24% for 1+, 53% for 2+, and 93% for 3+ or 4+.14 Another study reported a PV+ of 38% for ≥1+ proteinuria.15 A recent literature review concluded that the accuracy of 1+ proteinuria in pregnant women by dipstick was “poor and therefore of limited usefulness.”16 In a busy office with a number of healthy nonhypertensive women, the false-positive rate is high due to contamination with vaginal secretions, previous exercise, high specific gravity of urine, or other benign causes.17-19 In contrast to the high false-positive rates noted in the previous studies, Meyer et al reported a negative predictive value of only 34% for trace or negative proteinuria in hospitalized women with hypertension in pregnancy.20 Proteinuria detected by dipstick using visual or automated testing alone is a poor indicator for true proteinuria although the automated method is the more accurate of the 2.14 When the measurement of proteinuria is indicated for the early identification of preeclampsia, then a random protein:creatinine ratio is a better test choice.14,15,21
Three studies have addressed the question: Is proteinuria an accurate predictor for preeclampsia?6,15,16 Preeclampsia is defined as an elevated blood pressure with either proteinuria or edema or both.15
In a prospective observational study carried out in Australia, 866 non-hypertensive women were tested using an automated dipstick method for proteinuria on their first prenatal visit and 35 were ≥1+ positive.13 Twenty-five (71%) of these women had proteinuria detected during subsequent visits, and 2 (6%) of them developed preeclampsia. Of the 833 women who did not have proteinuria on the first visit, 316 had it on sub-sequent dipstick testing, and 15 of these women developed preeclampsia. Of the 512 who never had proteinuria, 9 developed preeclampsia (sensitivity=63%, PV–=98%). Proteinuria at the first visit may be a risk factor for subsequent preeclampsia (relative risk=2.2; 95% CI, 0.49–9.6]). Of the 8 women who developed proteinuria before hypertension developed, 5 could be considered at high risk: 2 had proteinuria at their first prenatal visit, 2 had multiple gestations, and 1 had a history of preeclampsia. Pregnancy outcomes were similar in the proteinuria and no proteinuria groups. The authors recommended discontinuing urine protein testing except in high-risk women (TABLE 2).
A retrospective study of 3104 low-risk American women which excluded those at high risk (multiple gestations, diabetes mellitus, preexisting hypertension, renal disease, or ≥30 mg/dL [1+] proteinuria at the first prenatal visit) found routine visually evaluated dipstick determination for proteinuria of no value in the prediction of preeclampsia.22 In this study for the 6.1% of woman who had a blood pressure of greater than 140/90 mm Hg, a weight gain of 3 pounds a week or more, or greater than 1+ edema, testing for proteinuria was considered to be for diagnostic reasons. When the remaining 2802 patients were evaluated throughout their pregnancy, 90.3% had no proteinuria, 7.6 % were 1+, and 2.2% were ≥2+. The sensitivity and PV+ of proteinuria for preeclampsia in routine patients were 5% and 96% respectively.